1. Field of the Invention
The present invention relates to a non-sintered nickel electrode for an alkaline storage battery which comprises an active material powder made up of composite particles, and to an alkaline storage battery including the non-sintered nickel electrode as its positive electrode, and more specifically to the improvement of the composite particles for the purpose of providing a non-sintered nickel electrode for an alkaline storage battery, and an alkaline storage battery including the electrode as its positive electrode, having a high active material utilization rate not only when charged at normal temperatures but also when charged at high temperatures, and having good charge-discharge characteristics.
2. Description of the Prior Art
As a positive electrode for a nickel-hydrogen storage battery or a nickel-cadmium storage battery, a sintered nickel electrode fabricated by sintering a nickel powder onto a perforated steel substrate and the like and impregnating the resulting plaque with an active material (nickel hydroxide) has been well known.
To ensure an increased impregnation with the active material for a sintered nickel electrode, it is necessary to employ a sintered substrate of increased porosity. However, since the inter-particle bond of sintered nickel is weak, increasing the porosity of the substrate enhances the tendency for the nickel particles to be dislodged from the sintered substrate. For practical purposes, therefore, the porosity of the sintered substrate cannot be increased beyond 80%, so that the sintered nickel electrode has the drawback that the impregnation amount of active material is limited. In addition, since the pore size of the sintered nickel is generally as small as 10 .mu.m or less, the impregnation of the sintered substrate with the active material requires the time-consuming dip method which involves several immersion cycles.
For the above reasons, a non-sintered nickel electrode, represented by a paste-type nickel electrode, has been proposed recently. The paste-type nickel electrode is fabricated by impregnating a high-porosity substrate (with a porosity of 95% or greater) with a paste prepared by kneading an active material (nickel hydroxide) and a binder (such as an aqueous solution of methyl cellulose) together. Since a high-porosity substrate can be used for the paste-type nickel electrode, not only the impregnation amount of active material can be increased, but also the impregnation procedure is facilitated.
However, when such a high-porosity substrate is used for increasing the impregnation amount of active material in a paste-type nickel electrode, the conductive capacity of the substrate as a current collector deteriorates, so that the active material utilization rate, i.e. utilization efficiency of active material is decreased. Moreover, when charging is carried out at high temperatures, since an oxygen-generating reaction takes place as a side reaction, nickel hydroxide cannot be oxidized to nickel oxyhydroxide sufficiently, and thus the active material utilization rate is decreased.
In order to increase the active material utilization rate, there has been proposed a paste-type nickel electrode using a composite active material consisting of nickel hydroxide, zinc or a zinc compound (such as ZnO), cobalt or a cobalt compound (such as CoO and Co(OH).sub.2) and a bismuth compound (such as Bi.sub.2 O.sub.3) (see JP-A-8-195198). The addition of a bismuth compound was intended to increase the oxygen overpotential of the nickel electrode at high temperatures, and to improve the active material utilization rate when charging is carried out at high temperatures.
However, investigations made by the present inventors revealed the drawback of the above conventional paste-type nickel electrode that the active material utilization rate is insufficient when charging is carried out at high temperatures, and that charge-discharge cycle characteristics are poor since the effect by the addition of a bismuth compound is lowered rapidly in charge-discharge cycles, that is, the capacity lowers in a short period of time.
Therefore, it is a primary object of the present invention to provide a non-sintered nickel electrode for an alkaline storage battery having a high active material utilization rate not only when charging is carried out at normal temperatures but also when it is carried out at high temperatures, and still having good charge-discharge cycle characteristics, and to provide an alkaline storage battery including such a non-sintered nickel electrode as its positive electrode.